This invention relates generally to optical spectrum analysis and, in particular, to a signal analyzer using tapped optical fibers, preferably with phase compensation.
Various gratings and interferometers have been used to extract the power spectrum of light carried by an optical fiber, including gratings embedded within the core of the fiber itself. As one example, U.S. Pat. No. 5,850,302 discloses an optical waveguide communication system including an optical fiber having a refractive index grating and coupling means selected such that at least a portion of the light is transferred from a guided mode into a radiation mode and is available for utilization by utilization means (e.g., a detector) outside of the waveguide and the coupling means. The optical fiber comprises a chirped and blazed refractive index Bragg grating selected such that at least a portion of the light in the guided mode is transferred into a non-guided mode.
The article further comprises utilization means for utilizing the light in the non-guided mode. In one embodiment, a conventional optical coupler is used to couple a small fraction (e.g., 5%) of multichannel signal power from the fiber. The coupled-out light propagates through the fiber to Fabry-Perot filter. The filter is selected to have very narrow transmission bands which pass through the filter and propagates to a dispersive waveguide tap (DWT) where the spectrum is spatially dispersed and detected. According to the patent, the DWT may be a tap in the fiber to provide increased spatial separation of the various wavelengths.
However, all existing techniques of this kind use a much more limited range of time delay and, hence, have poorer resolution and a more limited time-bandwidth-product. Current interferometer arrangements may need to use a scanning in one of the interferometer legs and hence do not give the entire spectrum at one time. Thus, the need remains for an approach that uses optical taps to provide a larger time-bandwidth-product, particularly for spectrum analysis purposes.
This invention improves upon the prior art by taking advantage of two-dimension fiber delay radiator (FDR) to generate a large time-bandwidth-product spectrum that can not be done with conventional one-dimensional device. The approach also produces a much finer resolution than a conventional one-dimensional device and hence can analyze the fine spectral structure of signals.
In the preferred embodiment, the device is made with taps that are Bragg gratings orientated at 45 degrees to the fiber core, which is not true for current devices. Hence, the light for the inventive device comes directly out of the side of the fiber, linearly polarized. The side-firing configuration also simplifies the required optics.
The optical fiber is wound on a cylindrical-like form such that a number of loops of the fiber are available for making a number of taps on each loop. Taps are preferably generated along each loop of the fiber so that a small portion of the light propagating in the fiber will exit sideways from fiber at the taps. The taps are preferably tilted Bragg type gratings formed in the core of the fiber. A lens system is then used to capture the light from the taps and produce the Fourier Transform of the total distribution of light from all the taps. A video camera then captures this Fourier Transform light and the power spectrum of the light signal is displayed on a monitor.
The preferred construction of the FDR would produce taps with perfect phase characteristics such that the arrangement described above would suffice. However, this is probably not practical to generate or maintain such taps. Hence, alternative embodiments are used and described herein. One includes a phase spatial light modulator to correct and modify the tap phases. Another uses a coherent reference wave to generate a holographic optical element (or complex spatial light modulator) to correct the tap phases and amplitudes. The architecture for which we have experimentally demonstrated the operation of the analyzer uses a digital holographic technique to correct for tap phases. In this technique, a coherent reference source and a detector array are used to capture the radiation amplitude pattern of the FDR. Then, with digital processing of the captured pattern, the desired spectral signal properties are obtained.